In general, aluminum castings are produced by more than a few casting processes depending on economic considerations, quality requirements and technical considerations. Although there are many specialized casting processes, including investment casting (also called lost wax), lost foam casting, centrifugal casting, plaster mold casting, ceramic mold casting, squeeze casting, semi-solid casting, and its variate slurry-on-demand casting, the three main casting processes are sand casting, permanent mold casting and high pressure die casting.
Sand Casting uses insulating sand molds resulting in a relatively slow cooling rate. The microstructural features, such as grain size or the aluminum dendritic arm spacing, are relatively large with the expectation that mechanical properties are lower because of the inverse relationship between the size of microstructural features and mechanical properties. Because of these features and properties, the quality of the casting is considered relatively low. Very small and very large castings up to several tons can be produced in sand casting in quantities ranging from only one to a few thousand. In high volume scenarios, sand castings are the most expensive because the sand mold has to be replicated for every casting. In low volume scenarios, the tooling cost per part is lower for sand casting than it is for permanent mold or high pressure die casting.
Permanent mold casting (whether gravity or low pressure) uses a metal mold or die with a coating to provide a barrier between the steel die and molten aluminum alloys to control and limit the heat extraction from the molten metal. Because of the variable thickness of the coating, the coating is frequently also responsible for a non-chemical sticking of the casting in the coated die requiring human intervention or monitoring as the casting is extracted from the die. Thus, the low pressure permanent mold process is not fully automated, unlike high pressure die casting. In some instances, water lines in the dies are used to control and increase heat extraction. The water can be provided at a given temperature and at a given flow rate or alternatively oil can be substituted for the water. As a result, when compared with the sand casting slow cooling rates, the permanent mold cooling rates are significantly higher, resulting in premium quality castings with smaller grain size, smaller aluminum dendrite arm spacing, and higher mechanical properties. In permanent mold casting, medium size castings up to 100 kg may be produced in quantities of from 1,000 to 100,000. As a result, cost on a per pound basis is lower cost than a sand casting because the albeit expensive permanent mold tooling may be used to make 100,000 castings or more. The steel dies are coated with a coating to prevent the molten alloy from soldering to the die during the casting process. The coating on the dies produces a surface finish on the casting that replicates the rough, undesirable topography of the coating. This rough finish often requires a secondary operation to obtain a smoother surface finish. In low pressure permanent mold casting, a molten alloy is pushed into the mold in the range of 3-15 psi.
Permanent mold casting (whether gravity or low pressure) produces parts with the highest mechanical properties because it is the only casting process that permits an economical, full T6 heat treatment. This solution heat treatment results in a homogenized microstructure while avoiding blistering. In high pressure die casting, solution heat treating times and temperatures must be significantly lowered to avoid “blistering” from trapped die release agents or air. In sand casting, by contrast, longer solution heat treating times and temperatures must be applied to homogenize the otherwise coarse microstructure and obtain the highest mechanical properties after solution heat treating and artificial aging. The surface finish in permanent mold casting, however, does not match the surface smoothness of either sand casting or die casting because the coating on the dies in permanent mold casting replicates the rough topography of the coating.
High pressure die casting uses uncoated dies and injects molten metal at high velocities into a die cavity with pressure intensification on the molten metal during solidification. Partly because of the turbulent filling, but primarily because of the high iron content (of about 1%) required for die soldering resistance, the quality of die castings and the mechanical properties of die castings are lower than both permanent mold casting and sand castings, despite the smaller grain size and smaller aluminum dendrite arm spacing. High pressure die castings are typically small castings up to about 50 kg. The tooling for high pressure die casting is expensive and is expected to produce large quantities of castings in the range of 10,000 to 100,000. Thus, the cost per pound of high pressure die castings are lower than permanent mold or sand casting.
Structural aluminum die casting refers to high pressure die casting with a low iron content. In structural aluminum die casting, high levels of manganese are typically used instead of iron to provide die soldering resistance. The Silafont™-36 alloy uses a manganese maximum of 0.80%, while the Aural™-2 alloy and Aural™-3 alloy both use a manganese maximum of 0.60%. Conventional copper containing Aluminum Association registered die casting alloys 380, A380, B380, C380, D380, E380, 381, 383, A383, B383, 384, A384, B384, and C384 all contain a manganese maximum of 0.50%, and are considered low quality alloys made from scrap. These lowest quality die casting alloys cannot be used as structural aluminum die casting alloys because the manganese is too high. It is commonly believed that manganese is the most important element in any die casting alloy because the manganese determines the iron level below which Mn/Fe-intermetallics do not form, according to quaternary Al-Si-Fe-Mn phase diagrams from the reference Solidification Characteristics of Aluminum Alloys, Vol. 2—Foundry Alloys by Lennard Backerud, Guocai Chai, Jamo Tamminen, 1990 AFS Book. At 0.1% manganese, the iron should be less than 0.7% to avoid the primary precipitation of intermetallics that decrease mechanical properties, particularly the ductility. Thus, to avoid the primary precipitation of intermetallics at 0.2% Mn, the iron should be less than 0.6%; at 0.3% Mn, the iron should be less than 0.5%; at 0.4% Mn, the iron should be less than 0.4%; at 0.5% Mn, the iron should be less than 0.3%; at 0.6% Mn, the iron should be less than 0.2%; at 0.7% Mn, the iron should be less than 0.1%; and finally at 0.8% Mn, the iron should be less than 0%—an impossibility. None of the conventional die casting alloys noted above meets the manganese and iron requirements to avoid the primary precipitation of intermetallics. Further, this means the Silafont™-36 alloy at 0.8% Mn with an Aluminum Association specification limit for iron at 0.12% Fe (which is quite low), will still precipitate intermetallics that decrease ductility. However, the Aural™-2 alloy and Aural™-3 alloy at 0.6% Mn with an Aluminum Association specification limit for iron at 0.25% may have a lesser tendency to precipitate intermetallics than the Silafont™-36 alloy because the iron limit to avoid the primary precipitation is below 0.20% when Mn is 0.6%.
This die soldering solution for high pressure die casting does not work for the low pressure permanent mold casting process. This is because iron and/or manganese, which is used exclusively in high pressure die casting for die soldering resistance (at bulk levels as high as 1.3% and 2%), cannot be used for die soldering resistance in the slower cooling, low pressure permanent mold casting process, because the primary precipitated intermetallics would grow larger during solidification than in die casting and have a more significant effect on decreasing mechanical properties.